There is no admission that the background art disclosed in this section constitutes prior art.
In magnetic lenses of the kind employed in various charged particle beam apparatus, and particularly with respect to electron-beam-comprising apparatus, a magnetic flux is created by flowing a current through a coil. This generates heat as a by-product. In the semiconductor industry, for example and not by way of limitation, electron beams are used to direct write a pattern on device structures, to pattern lithographic masks, to monitor the alignment of patterns during device manufacturing, and in inspection tools which evaluate device structure to determine whether a specification is met, for example. Due to the size of semiconductor devices today, there frequently is a requirement that an electron beam landing position be controlled to within one nm. This is very difficult to do with respect to many of the apparatus which make use of an electron beam.
The power dissipated in each magnetic lens is typically in a range between 10 W and 100 W. This leads to temperature changes in materials in the area of the magnetic lenses, especially in pole pieces of an electron beam apparatus, which in turn causes changes in mechanical dimensions due to thermal expansion. These changes in mechanical dimensions frequently lead to instabilities of the charged particle beam, such as an electron beam, especially if the current flowing through the coils is changed during operation of the charged particle beam. To prevent this instability, state-or-the-art electron beam column magnetic lenses are cooled. This cooling is generally done using conductive heat transfer from the magnetic lens coils. The conductive heat transfer is typically achieved using cooling tubes in contact with the magnetic lens coils, where a fluid, typically water, is flowed through the cooling tubes. However, integration of the cooling tubes in a manner which provides for uniform removal of heat from the magnetic lenses is difficult, due to the mechanical complexity of the magnetic lenses. Further, the volume of the magnetic lenses is increased by the addition of cooling tubes. Minimization of the volume is an important aspect in magnetic lens design. Other disadvantages in the use of cooling tubes include the problems related to obtaining good thermal contact between the tubes and the magnetic coils and the possibility of a leak developing in a cooling tube.
U.S. Pat. No. 5,012,104 to Lydia J. Young, issued Apr. 30, 1991, describes what is said to be a thermally stable magnetic deflection assembly for use in an electron or particle beam machine with several separately potted magnetic coils spaced apart and arranged in a particular vertical position on a central pipe. Non metallic, low coefficient of thermal expansion, highly thermally conductive materials are sued throughout and means are provided in an effort to maintain the entire assembly at a desired temperature. (Abstract) FIG. 3 of the patent shows how radial walls of a magnetic wound coil are encased by a potting material. The potted coils are then stacked on a pipe formed of a not-metallic, thermally stable material such as ceramic, which, when positioned in a beam column, surround the central tube. A three piece shroud of non metallic material surrounds the coils to provide a flow path for coolant which is in contact with the exterior of the potting material.
U.S. Pat. No. 5,264,706 to Oae et al., issued Nov. 23, 1993, discloses an electron beam exposure system which includes an electron beam along an optical axis and an evacuated column for accommodating the beam source and extending along the optical axis. Also included are an electron lens for focusing the electron beam and an electromagnetic deflector supplied with a control signal for deflecting the electron beam in response to the control signal. The electromagnetic deflector comprises an inner sleeve surrounding an evacuated column, and first and second mutually separate windings provided on an outer surface of the inner sleeve in an opposed relationship with respect to each other across the optical axis of the electron beam source. An outer sleeve surrounds the inner sleeve with a separation between the sleeves defining a passageway for the flow of coolant. (Abstract) Coolant enters at the bottom of the electron beam column and exits at the top of the column.
U.S. Pat. No. 5,382,800 to Nishino et al., issued Jan. 17, 1995 describes a charged particle beam exposure method for deflecting a charged particle beam in a deflection system which includes electromagnetic deflection coils. The method includes the steps of controlling the deflection system based on deflection data, and generating heat at least in a vicinity of the electromagnetic deflection coils so as to compensate for a change in heat generated from the electromagnetic deflection coils. (Abstract).
U.S. Pat. No. 6,053,241 to Kendall et al., issued Apr. 25, 2000, describes a method of cooling a deflection system for a particle beam, where vibration sensitive devices are contained in the system. The method comprises providing a vibrating cooled heat exchange structure spaced away from the vibration sensitive deflection devices. The technique used is transmission of the heat away from the vibration sensitive devices to the heat exchange structure through a high thermal conductivity structure such as a cold plate. The heat is transmitted from a static heat exchange structure with a static, inert fluid through cold plates to a vibrating heat exchanger cooled by turbulent liquid passing through a coil in the heat exchanger.
The references discussed above show the concern with the effect of heat generation by electromagnetic coils which are used as part of a particle beam system. This concern has increased in recent years as the feature size to be created by or measured by the particle beam has decreased and the error tolerable in positioning of the particle beam optical axis has decreased. For example, current day requirements for an electron beam are that the beam axis position be controlled to a nominal value in the range of about 1 nm. The past requirements for position control were in the range of about 5 nm to about 10 nm. This change in the beam optical axis position control requirement has caused re-evaluation of cooling requirements, in an effort to reduce a drift in the beam optical axis position caused by expansion of apparatus elements in the particle beam column. Our empirical testing has shown that it is possible to meet the 1 nm requirement by improving the method of heat transfer from elements of the particle beam apparatus, and by improving the ability of the cooling device to remove heat from the apparatus elements which affect control over the position of the optical axis of the beam.